On The Distributions And Controls Of Stable Barium Isotopic Compositions In The Pearl River Estuary And Its Potential As A New Proxy

Barium(Ba)is a widely used proxy in oceanography for tracing water mass mixing,river inputs,and biological productivity.As a transition zone between land and sea,estuaries can act as an efficient filter of riverine trace elements before they reach the ocean.However,the exchange between sediment-water interface can release these elements back into the water column,potentially affecting the estuarine mass balance of elements and their riverine fluxes to the ocean.The geochemical behavior of Ba in estuaries has been studied for more than 50 years,but several scientific problems remain elusive.For example,the exact mechanisms inducing a maximum concentration of dissolved barium(DBamax)at low salinities in estuaries are not fully constrained.The prerequisite for using Ba as a paleosalinity proxy,i.e.,an apparently conservative Ba-salinity relationship after DBamax,is likely interfered by seasonal variations and benthic inputs.The use of sedimentary Ba as a paleoproductivity proxy is possibly compromised by barite dissolution in sulfate-depleted sediments.On the other hand,the stable isotopic compositions record information on a longer time scale than elemental concentrations,and the fractionation effect of isotopes can better distinguish the influence between different biochemical processes and physical mixing.Enabled by the recent success in the determination of stable Ba isotopic compositions(?138/134Ba)in seawater,Ba isotopes have been suggested as a novel tool to study the biogeochemical cycling of Ba.This dissertation,for the first time,systematically investigated the distributions and controls of ?138/134Ba in the water column and sediment pore-water of the Pearl River Estuary(PRE),and preliminarily explored the potential of Ba isotopes as a new proxy of paleosalinity and paleoproductivity.Surface DBa concentrations in both summer(July 2015)and winter(January 2017)in the PRE showed overall non-conservative distributions along salinity gradients.A distinct DBamax was observed near the Humen outlet in both seasons with values of 224.7 and 230.3 nmol kg-1 corresponding to salinities of 2.3 and 4.9,respectively.After DBamax,DBa concentrations decreased linearly with increasing salinity till the sea end.Instead of being affected by tributary inputs from Xijiang,Beijiang,and Dongjiang,the surface DBamax mainly resulted from Ba addition during the particle desorption as suggested by simulation results of an ion exchange model.The benthic input via for example pore-water exchange might also add a few portion.?138/134Ba values generally increased from the river endmember(?0.20‰)to the seawater endmember(?0.60‰),roughly mirroring the distribution of DBa concentrations.Surface ?138/134Ba was slightly lighter at the DBamax,in particular in winter(-0.15‰).?138/134Ba also behaved conservatively against salinity after the DBamax,suggesting that Ba isotopes are a potential new tracer of paleosalinity in proximity to large river plumes.We speculate that particle adsorption of Ba preferentially removes lighter Ba isotopes,resulting in lower DBa concentration and heavier ?138/134Ba in the river endmember at zero salinity of the PRE.Based on a Rayleigh or a steady state model,the fractionation factor(?138/134Ba)in this process was estimated to be-0.3‰ to-0.5‰.In January 2017,pore-water Ba concentration and Ba isotopic composition data were collected from four stations from upper reaches to the lower seaside of the PRE.We find pronounced depth-dependent ?138/134Ba variations generally showing decrease from the topmost layer down to 15 cm depth,except that at one station exhibited a?138/134Ba minimum was observed at 6 cm corresponding to a Ba concentration maximum.These gradients,primarily resulting from Ba isotope fractionation caused by various solution-solid phase interactions,confirm that Ba isotopes are potentially a powerful tracer for Ba dynamics during sediment diagenesis.Both formation of authigenic barite and Ba precipitation associated with the redox cycling of manganese preferentially removes light Ba isotopes from the pore-water with comparable apparent ?138/134Ba of-0.3‰.Pore-water Ba concentrations at the four stations are significantly higher than those in the corresponding bottom water,stimulating a benthic flux of Ba into the overlying water.However,Pore-water ?138/134Ba in the topmost layer was overall heavier than in the bottom water.The heavy pore-water Ba isotopic signatures were thus not significantly imprinted in the water column,which was likely diluted by other isotopically light sources in the sediment-water interface.A conservative mixing model of DBa and ?138/134Ba showed Ba isotope distributions in the bottom water of the PRE were dominated by two-endmember mixing between the river water and seawater.Ba isotopes in sediments are thus potentially a more robust proxy of paleosalinity than Ba concentrations because considerable benthic fluxes of Ba would not significantly influence a conservative ?138/134 Ba-salinity relationship in the water column of estuaries.On the other hand,barite dissolution in sulfate-depleted sediments and subsequent precipitation of authigenic barite highly likely alter the original sedimentary Ba isotopic compositions,implying that in this case using Ba isotopes as a paleoproductivity proxy should be considered with caution.To sum up,stable Ba isotopic compositions in both the surface water and the sediment pore-water of the PRE displayed significant spatial variability,mainly resulting from various geochemical processes during estuarine mixing or early diagenesis.As a consequence,Ba isotopes can be used to farther constrain the oceanic Ba cycle and applied as a new proxy in paleoceanography and paleoclimatology.